1. Field of the Invention
The present invention relates to a transverse electric field mode liquid crystal display device. In particular, the present invention relates to improvements in the display of black color at an oblique viewing angle.
2. Description of the Related Art
In a liquid crystal display panel, the light transmittance is adjusted by applying voltage to the liquid crystal to control the alignment of the liquid crystal. Liquid crystal display modes are classified according to their types of the alignment changes of liquid crystal. Currently, major modes include TN (Twisted Nematic) mode, VA (Vertical Alignment) mode, and IPS (In-Plane Switching) mode.
In the TN mode, a longitudinal electric field is vertically applied to the substrate to make the liquid crystal molecules stand up or lie down with respect to the substrate surface in response to the applied voltage, in order to change the display state. In the VA mode, the display state is changed by causing the movements of the liquid crystal molecules with respect to the substrate surface in a similar manner.
In the IPS mode, a transverse electric field is horizontally applied to the substrate to make the movements of the liquid crystal molecules. The modes in which the movements of the liquid crystal molecules are made by the transverse electric field include FFS (Fringe Field Switching) mode, which is a kind of the IPS mode, in addition to the ordinary IPS mode. Since the movements of the liquid crystal molecules are made in the plane parallel to the substrate to change the ON and OFF states of the display state in the transverse electric field modes such as the IPS mode and FFS mode, these modes are characterized that they have a smaller retardation (Δn·d) change, i.e., a smaller retardation caused by viewing angle change, of the liquid crystal layer, and a wider viewing angle. The retardation Δn·d is defined as the product of the refractive index anisotropy Δn of liquid crystal molecules and a liquid crystal layer thickness d.
The viewing angle characteristic of the IPS mode is significantly better than that of the TN mode and VA mode. In general, an IPS mode liquid crystal display device without a viewing angle compensation film has a better viewing angle characteristic than that of TN mode or VA mode liquid crystal display device having an optimal viewing angle compensation film.
As described above, a good viewing characteristic can be achieved in a liquid crystal display device by minimizing the viewing angle dependence of the liquid crystal layer itself. However, there is another factor affecting the viewing angle characteristic besides the viewing angle dependence of the liquid crystal layer itself. It is the viewing angle dependence of a pair of orthogonally-arranged polarizing plates (crossed Nichol polarizing plates). Even in the pair of orthogonal polarizing plates alone, the orthogonality becomes out of order when viewed from an oblique angle. As a result, even without the liquid crystal layer, light leak occurs in black display at an oblique viewing angle, and thereby the resulting deterioration of the viewing angle characteristic becomes problematic.
The level of the light leak of the polarizing plate itself in black display is small in comparison with the light leak caused by the retardation change of the liquid crystal layer. Therefore, it has not been viewed as a problem in those modes that has larger light leak caused by the liquid crystal layer itself, such as TN mode. However, it has become problematic for IPS mode and FFS mode, which have an excellent viewing angle characteristic of the liquid crystal layer itself.
In order to solve the problem that light leak occurs due to the disordered orthogonality in the pair of polarizing plates in black display at an oblique viewing angle and thereby the viewing angle characteristic is deteriorated, some methods have been proposed in which an optical compensation layer having a birefringence characteristic is interposed between the liquid crystal layer and the polarizer (for example, Patent documents 1-4).
Japanese Unexamined Patent Application Publication No. 2001-350022 (Patent document 1) proposes a method of implementing viewing angle compensation for crossed Nichol polarizing plates, in which two biaxial optical compensation films having ½ wavelength retardation are interposed between the substrate and the polarizer in order to make the polarization state at oblique incidence equal to the polarization state at vertical incidence. Japanese Unexamined Patent Application Publication No. 11-305217 (Patent document 2) proposes a method in which one biaxial optical compensation film having ½ wavelength retardation is interposed between the substrate and polarizing plate. Japanese Unexamined Patent Application Publication No. 2005-321528 (Patent document 3) discloses a structure in which a first retardation region having a retardation layer, which is formed by stretching an alicyclic structure-containing polymer resin film, and a second retardation region are arranged between the substrate and the polarizing film (polarizer). Japanese Unexamined Patent Application Publication No. 2005-196119 (Patent document 4) proposes a structure in which an optical compensation layer having a compound containing discotic structure units, and a protective film are interposed between the substrate and the polarizing film (polarizer).
Furthermore, Patent document 4 also discloses a structure to improve the viewing angle characteristic without using the above-mentioned optical compensation layer having a compound containing discotic structure units, by providing a polarizer sandwiched between a pair of protective films.